1. Field of the Invention
The present invention relates generally to a system for measuring an intake air amount to be introduced into an engine combustion chamber. More specifically, the invention relates to an intake air amount measuring system which can derive an intake air amount to be introduced into an engine combustion chamber with high precision and which system is suitable to be coupled with a high precision engine control systems for providing precise intake air amount data.
2. Description of the Background Art
In an electronic or electric engine control systems, such as a fuel injection system, idling control system and so forth, for controlling engine operation for optimizing the engine performance, amount of air to be introduced into an engine combustion chamber is regarded as one of the essential parameters. Conventionally, the intake air amount has been measured by means of air flow meters, such as flap type air flow meter, hot wire type air flow meter, Karman vortex type air flow meter and so forth. In the alternative, the air flow amount can also measured by measuring a vacuum pressure in an air induction system.
Such conventional techniques in measurement of intake air amount tends to be influenced by pulsatile air flow through the air induction system. Therefore, such conventional techniques are not as precise as that required for high precision engine control which has been required nowadays.
In this view, there has been proposed a new technology for measuring intake air amount based on a throttle valve angular position which represents air flow path area in the induction system, and an engine speed. Such new technology has been proposed in the Japanese Utility Model Second (examined) Publication (Jikko) Showa 60-39465. In the disclosed system, intake air volume is arithmetically derived on the basis of the throttle valve angular position and the engine speed and intake air weight is derived on the basis of the intake air volume and intake air density which is variable depending upon the temperature of the intake air. In order to monitor the intake air temperature, an intake air temperature sensor is disposed in the air induction system upstream of a throttle valve. The disclosed system derives the air density on the basis of the intake air temperature monitored by the intake air temperature sensor. This may achieve substantial improvement in precision of measurement of an intake air amount.
However, this system does not concern about variation of the air density caused downstream of the sensor due to temperature variation caused by various factors. For example, in some of the air induction system, the intake air is heated by means of an engine coolant recirculated from the engine cooling chamber for obtaining better fuel atomization characteristics. In such case, the intake air temperature at downstream of the intake air temperature sensor becomes higher than that measured to cause lowering of the intake air density. Alternatively, because of adiabatic expansion of the intake air is caused at the throttle valve, the intake air temperature tends to be lowered to increase the intake air density. In addition, in case of a single-point injection system, fuel is injected into the air induction system upstream of the throttle valve so that the injected fuel can be atomized utilizing the jet flow through the throttle valve. Atomization of the fuel naturally lower the temperature of the intake air to cause increasing of the air density.
Therefore, when the intake air amount as monitored by the intake air measuring system as disclosed in the aforementioned Tokko Showa 60-39465, the intake air amount data to be used for engine control, such as fuel injection control, spark ignition timing control and so forth, cannot precisely correspond to that actually introduced into the engine combustion chamber. As a result, the engine control to be performed with taking the intake air amount data derived in the prior proposed process, cannot be satisfactorily precise as that required in nowadays.